The present invention relates to a high frequency heating apparatus, more particularly, to a high frequency heating apparatus provided with a damper which is opened or closed by using a negative thermal expansion metal such as a shape memory alloy.
Recently, a high frequency heating apparatus has been generally used which functions as both a microwave heating oven and a convection heating oven for recirculating heated air in a heating chamber to cook the foodstuff.
In such a high frequency heating apparatus, gas associated with steam or smells or produced from heated foodstuff in the heating chamber is ventilated from the heating chamber to the outside by air cooling electrical components such as a magnetron or the like in the mode of the microwave heating oven. In the mode of the convection heating oven, the air which is introduced into the heating chamber must be sealed to effectively raise the temperature in the heating chamber, by closing a damper.
The conventional damper is opened and closed by an operating lever provided on an operating panel, or automatically by a solenoid or the like.
Further, recently, a damper mechanism which is operated by using a spring made of a shape memory alloy has been proposed as disclosed in U.S. patent application Ser. No. 484,460, filed on Apr. 13, 1983 by M. TANIGAWA, entitled "MICROWAVE OVEN DAMPER MECHANISM ACTIVATED BY A SHAPE MEMORY ALLOY". The U.K. Pat. counterpart was filed on Apr. 12, 1983 as No. 8309895. The German Pat. counterpart was filed on Apr. 19, 1983 as No. P3314055.3. Usually, it is necessary to supply a power such as low voltage and a high current in the shape memory alloy in order to change its shape. Accordingly, a transformer for reducing the voltage must be provided for driving the shape memory alloy, and a power circuit is additionally required for precluding the feeding of current to the shape memory alloy in order to continuously open the damper in the microwave heating mode, or for feeding the current directly to the shape memory alloy in order to continuously close the damper in the convection heating mode. Since the transformer and the power circuit need many components, a circuit for the apparatus is not simple.